1. Field of Invention
The present invention relates to the field of downhole tools. More specifically, the invention relates to a device and method for use in a downhole well tool having a hydraulic piston and cylinder assembly.
2. Related Art
Subsurface safety valves are positioned in a well to allow control of flow to the surface, particularly during a blowout, to avoid damage to people, the environment, and equipment and to avoid loss of hydrocarbons. In one type of safety valve, the valve is opened by the application of hydraulic fluid from the well surface and closed by a biasing means, such as an enclosed pressure reference chamber and a mechanical spring.
Safety valves must close under all circumstances. If there is a failure of the safety valve, the safety valve must be failsafe in the closed position so that the valve closes during any failure of the seals or other valve components. As valves are set deeper, safety valves incorporate reference chambers having compressed gas for a biasing force in addition to a biasing spring as a way to overcome the force of the hydrostatic head. The gas acts against a piston area to create a closing force much higher than that obtainable with a conventional mechanical spring.
The piston is attached to the flow tube used to open and close the safety valve and is, therefore, at least partially exposed to the tubing pressure. Thus, to maintain the gas charge and the hydrostatic control on the piston, the piston includes seals isolating the various pressures applied to the piston. If the seals leak or fail, loss of the compressed biasing gas charge may reduce the available biasing force to a level that is insufficient to close the valve. Likewise, if the seals leak or fail, high pressure tubing gas may overcome the biasing gas pressure to prevent valve closure. To overcome the dangers associated with seal leak or failure, certain gas biased safety valves, such as those disclosed in U.S. Pat. Nos. 4,660,646 and 4,976,317, allow a valve to failsafe close if the gas charge is lost.
Prior failsafe gas biased subsurface safety valves typically require numerous parts and seals to operate. For example, previous designs typically require relatively complex release mechanisms to operate. Typically, the prior devices have a spring that has a relatively small biasing force and is used to bias the piston control valve of the piston to an open position. Another spring of the prior devices is used in a release mechanism that releases separate piston components in the event of a seal failure. The separation of the piston components facilitates equalization of the pressure above and below the piston allowing the spring acting on the flow tube to lift the flow tube and close the safety valve. This operation is described in U.S. Pat. No. 4,660,646 which uses the spring force of a collet as the "other" spring in the release mechanism. U.S. Pat. No. 4,976,317 discloses another embodiment that uses two mechanical springs including one in the release mechanism and one to bias the piston valve open.
The use of the multiple components adds complexity, length, and expense to the safety valve. Thus, despite the use of the prior art features, there remains a need for a gas biased subsurface safety valve that is simpler in design, more compact, and less costly relative to prior devices while providing the same failsafe features.
Additionally, due to the harsh environments of wells and the reliability requirements of safety valves, safety valves are typically made from relatively expensive materials, such as Inconel alloy 718. Also, safety valves are typically relatively long in order to accommodate all of the components required for operation. For example, present safety valves typically mount the piston, the filter, and the gas charge in stacked relation so that each occupies a separate axial length of the safety valve. Due to the relatively high cost of material, however, any reduction in the length of the safety valve results in substantial cost savings. Accordingly, there is a continuing need for shorter safety valves that perform the same functions of previous safety valves.
One consideration involved in the design of the safety valve involves maintaining a seal within and the packaging of the gas, or reference, charge, particularly the interface between the gas charge and the operating piston. Typically, the gas charge includes a liquid, such as an oil, between the gas charge and the piston to facilitate sealing and lubrication. One problem associated with such a system involves maintaining the gas/liquid interface at a position removed from the piston, particularly during shipping of the safety valve when the valve may be oriented in a variety of positions. One manner of addressing this problem is shown in U.S. Pat. No 4,976,317 which discloses the use of a reference chamber comprising a relatively small diameter tubing wrapped around the safety valve a plurality of times encircling the main bore and positioned within a separate compartment within the body of the valve. The small diameter combined with its length (provided by the plurality of times that the tubing is wrapped around the valve body) act to prevent the interface of the liquid and gas from reaching the piston. The wrapping of the tubing also generally requires that the reference chamber be positioned within a separate axial length from the operating piston. Accordingly, there is also a need for a system that provides the advantages of the prior system pertaining to the gas charge gas/liquid interface and that eliminates the need for the gas charge to be positioned within a separate axial length from the operating piston.